Because of their particular combination of properties such as transparency, heat-resistance and dimensional stability, polycarbonates are generally used as materials for the injection or injection-compression molding of optical data storage media. Additives, such as mold release agents and stabilizers are generally added to the polycarbonate in order to improve processability, processing generally proceeding at temperatures in the range from 300° C. to 400° C.
Aromatic polycarbonates based on bisphenol A are in particular used for the production of optical data storage media. They are, however, capable of absorbing up to 0.34 wt. % of water, which may have an unfavorable impact on the dimensional stability of the data storage media. Improved dimensional stability is, however, of significance, particularly when using blue or blue-green lasers.
U.S. Pat. No. 6,391,418 describes substrates for data storage media, which contain a biphenyl derivative as an additive to increase dimensional stability (lower shrinkage).
The addition of small quantities of m-terphenyl to bisphenol A polycarbonate is described in M. Ueda, Mitsubishi Engineering Plastics Corp., Technical Digest of Joint ISOM/ODS 2002 Waikoloa Hi., 8.7.2002, pages 33-35, this addition resulting in a reduction of water absorption. These biphenyl derivatives have the disadvantage, however, that they are highly conjugated aromatic π-systems, which exhibit absorption in the blue or blue-green range of the spectrum. This is undesirable in storage technologies which operate in this wavelength range. Terphenyls are moreover relatively rigid molecules, the rigidity having a negative effect on mechanical properties in a mixture with polycarbonate.
The options described in the prior art thus do not provide results which are satisfactory in every respect. There is, however, no indication to be found in the prior art that cyclic oligoformals could be suitable as additives.
The object accordingly arose of providing thermoplastic molding compositions containing polycarbonate with reduced water absorption which consequently exhibit better dimensional stability. New disk formats, which have a higher storage capacity and may also be thinner, such as for example digital versatile disks (DVDs), in particular require greater thermal stability in comparison with CDs. Any material damage occurring during conversion into moldings and the formation of deposits in the mold are becoming more critical. It is accordingly desirable for an additive for reducing water absorption simultaneously to bring about a reduction in melt viscosity and thus better flow at somewhat lower temperatures.